The excellent fire resistance of the phenolic resin foam is attributed to the following reason. When, for example, the external wall of a house which is made from the phenolic resin foam is exposed to a high temperature owing to a fire that has broken out in a neighboring house, the phenolic resin foam as a heat insulating material is gradually oxidized at a temperature of over 200.degree. C. to form a semicrystalline porous matrix layer (char). The resulting char covers the surface of the substrate. This layer has the ability to inhibit combustion, and even when a large amount of oxygen is supplied to the surface of the char and the temperature of a foam becomes as high as more than 1,000.degree. C., the attack of the fire against the substrate of the phenolic resin foam does not easily proceed. When the surface of the phenolic resin foam is carbonized, it first becomes amorphous carbon having an ignition point of up to 300.degree. C. When it is exposed to higher temperatures, it is converted to a graphite crystal. The ignition point of graphite is 450.degree. to 550.degree. C. Thus, in the event of fire, the phenolic resin foam absorbs heat during exposure to high temperatures and becomes graphatized at its surface. This results in the formation of a graphite covering having a high ignition point, and prevents combustion from proceeding to the inside of the phenolic resin foam.
The phenolic resin foam is produced by uniformly stirring a composition composed of a liquid resole-type phenolic resin, a blowing agent (such as Freon, petroleum ether, butane, water, etc.), a cell stabilizer (such as a silicone oil and a surfactant (e.g., dodecyl benzene)), and an acid catalyst which is a curing agent (such as sulfuric acid, hydrochloric acid, phosphoric acid, benzenesulfonic acid, toluenesulfonic acid, phenolsulfonic acid, etc.), and as required, an inorganic filler, a pigment, etc., pouring the uniform mixture into a mold, and heating it at 25.degree. to 70.degree. C., preferably 40.degree. to 65.degree. C., to expand it.
The resole-type phenolic resin foam has better fire resistance and heat insulating property than urethane foam and polystyrene foam, but it is very fragile.
In an attempt to improve the friability resistance of such a phenolic resin foam, a method was proposed in which sodium metaborate tetrahydrate (NaBO.sub.2..sub.4 H.sub.2 O) or borax (Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O) is included in the phenolic resin foam and a ceramic layer is formed at a high temperature as disclosed in, for example, Japanese Patent Application (OPI) Nos. 57768/76 and 765/80 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). Since these hydrates evolve a water vapor at a temperature below the decomposition temperature of the phenolic resin foam, the foam has high fire resistance.
However, since the liquid resole resin as a material for the phenolic resin foam contains about 10 to 20% of water attributed to the starting formalin, sodium metaborate tetrahydrate or borax is alkaline in the liquid resole resin (a saturated aqueous solution of the former has a pH of 11.8, and a saturated aqueous solution of the latter has a pH of 9.3). Accordingly, in the production of a resin foam by incorporating such an alkaline hydrate in a liquid resole resin together with a blowing agent, a foam regulating agent, a curing agent, etc., the curing reaction of the resole resin does not take place (namely, a foam cannot be obtained) unless the acidic curing agent is present in an amount more than which is sufficient to neutralize the alkaline hydrate. Consequently, a fairly large amount of the acidic curing agent must be used. Furthermore, large amounts of acidic ions (such as SO.sub.4.sup.-- and Cl.sup.-) attributed to a salt formed between the hydrate and the curing agent remain, and accelerate corrosion of metallic building materials. Furthermore, since the curing agent such as sulfuric acid or hydrochloric acid is used as a dilution in water, the use of a large amount of the acidic curing agent inhibits the curing of the foamable ingredients because of the presence of a large amount of water, or may cause a deterioration in the mechanical properties of the resulting foam.
Specifically, 50% sulfuric acid (curing agent) is usually used in an amount of 5 to 20 g to cure 100 g of the resole resin. When 50 g of sodium metaborate is used per 100 g of the resole resin, the amount of 50% sulfuric acid required to neutralize sodium metaborate is about 34 g, as shown in the specification of Japanese Patent Application (OPI) No. 57768/76, and in addition to this amount, 5 to 20 g of 50% sulfuric acid is required as a curing agent.
The phenolic resin foam also have the defect of high thermal conductivity.